1. Field of the Invention
The present invention relates to a semiconductor optical amplifier for amplifying optical signals, and more particularly to a gain-clamped semiconductor optical amplifier (hereinafter, referred to as “SOA”) capable of providing substantially constant gain for output optical signals at all times.
2. Description of the Related Art
When reaching the gain saturation region, an SOA cannot perform its function as an amplifier due to a crosstalk effect that is generated among channels. In order to prevent such a gain saturation phenomenon, it has been used to induce lasing in the SOAs, so as to clamp the gain of the amplifiers.
The lasing apparatus, which are used to clamp the gain of the SOAs, are largely classified according to the method. One is the Distributed Feedback (DFB) method and the other is the Distributed Bragg Reflector (DBR) method.
FIG. 1 is a schematic construction of a conventional gain-clamped SOA employing the Distributed Feedback method. The SOA, in which the Distributed Feedback method is applied, has a structure incorporating a grating 2 on the underside of a active waveguide 1 in the amplifier. The structure may be constructed according to the same method as the construction method of a general SOA, except adding a grating 2 in the structure of the general SOA including a active waveguide 1 and clad layers 3.
In the Distributed Feedback method, a grating is formed under the active waveguide having electric carrier density and the photon density varies according to input of an electric current and an optical signal. Consequently, the effective grating pitch of the grating varies. The variation of the effective grating pitch of the grating causes instability of the laser formed by the grating. Therefore, because the gain property of the SOA is unstable, it is difficult to obtain a stable clamped gain property as originally purposed.
The Distributed Bragg Reflector method has a structure wherein (1) a passive waveguide is formed adjacent to the outer part of a active waveguide and (2) a grating is formed on the underside of the passive waveguide. In order to achieve this structure, the “butt-joint” connecting the passive waveguide and the active waveguide directly or a dual waveguide structure arraying a passive waveguide and a active waveguide side by side must be employed. However, this arrangement has a disadvantage in that the construction and manufacture of the waveguide structure are relatively difficult as compared to the Distributed Feedback method. Also, the optical coupling efficiency between the passive waveguide and the active waveguide largely affects the properties of the SOA.
As the Distributed Bragg Reflector method includes a grating formed on the underside of the passive waveguide, a variation of electron density from input of electric current is not generated. Thus, the effective grating pitch varies little. Therefore, a stable laser may be obtained and the gain property of the SOA is also stabilized. However, it is necessary to form a passive waveguide on the outer part of a active waveguide for such a structure, which has its difficulties.
In particular, there are two ways to form a passive waveguide on the outer part of a active waveguide as follows. First, as shown in FIG. 2, there is the butt-joint which forms and connects a passive waveguide 24 at the end part of a active waveguide 21. FIG. 2 is a schematic construction of a gain-clamped SOA employing the Distributed Bragg Reflector method using a butt-joint. In FIG. 2, reference number 22 designates a grating and reference number 23 designates clad layers.
However, the gain-clamped SOA of the Distributed Bragg Reflector method using a butt-joint has a number of limitations, for example, manufacturing is difficult, the reflection between the active waveguide and the passive waveguide is large, and the properties of the amplifier are degraded because of the optical coupling loss between the passive waveguide and the active waveguide.
Secondly, the other method to form a passive waveguide on the outer part of a active waveguide is to use a dual waveguide as shown in FIG. 3. The gain-clamped SOA using the dual waveguide, in which a active waveguide 31 and a passive waveguide 34 are formed side by side in a side view. This structure has active waveguide 31 positioned at central part with shorter length than passive waveguide 34. In operation, light passing through passive waveguide 34 moves to active waveguide 31 to obtain gain, and then moves again to passive waveguide 34 to pass. In FIG. 3, reference number 32 designates a grating and reference number 33 designates clad layers respectively.
However, because the dual waveguide structure also has limitations, including the optical coupling efficiency between the active waveguide and the passive waveguide, there is a problem in that the properties of the amplifier are adversely affected.